If you look at the top panel, you see that the back wheel is about as broad as the door and last window. In the lowest panel, the wheel is broader than the whole side of the carriage. It should be equally wide, in relation to the side of the carriage, as in the top panel.

I would construct a box and four circles in Google Sketchup to visualize the perspective in this. Or take some cardboard box, cut it to the correct width-height-depth ratio, draw cicles on the sides, and then take a photo from the right angle.